One type of device for printing on a container includes a transport element that rotates about a vertical axis. Such a device typically includes ink-jet printers having print heads.
An important factor for optically high-quality printing of containers is the most exact possible orientation of the print head relative to the container that is to be printed upon. As a result, print head calibration is important.
One way to spot calibration errors is to have two print heads print the same image. If the print heads are correctly calibrated, the images should be identical. Otherwise, one can infer a calibration error. A machine-vision system is useful for evaluating calibration in this way. Based on the detected error, one should, in principle, be able to adjust the print head to eliminate that error.
A disadvantage of the above method is that a print head can be adjusted with several degrees of freedom. In many cases, given a particular error, it is not always clear which type of adjustment should be made.